One thing I've always wondered is if there are any Sannin in the other major ninja countries. I'd like to think that there are, although this remains to be seen. If there are other Sannin I'd bet that Garra and his siblings will became the Sannin for the sand village.
Geographically we've learned very little about the Naruto world. Most of the other villages have remained a mistery so far so all we can do is speculate about what they may be like. These other villages also seem to have legedary figures. Like when Kisame mentions the seven mist swordsmen. I'm guessing one is Zambuza and he's another but there are still five other mysterious characters who we have not met.
It would be cool if the reason they earned their title is because they defended a small town the water country from insurmountable odds ala the classic film the seven samuri.